Chemical-mechanical polishing ("CMP") processes remove materials from the surface layer of a wafer in the production of ultra-high density. integrated circuits. In a typical CMP process, a wafer presses against a polishing pad in the presence of a slurry under controlled chemical, pressure, velocity, and temperature conditions. The slurry, solution has abrasive particles that abrade the surface of the wafer, and chemicals that oxidize and/or etch the surface of the wafer. Thus, when relative motion is imparted between the wafer and the pad, material is removed from the surface of the wafer by the abrasive particles (mechanical removal) and by the chemicals (chemical removal) in the slurry.
FIG. 1 schematically illustrates a conventional CMP machine 10 with a platen 20, a wafer carrier 30, a polishing pad 40, and a slurry 44 on the polishing pad. The platen 20 has a surface 22 upon which the polishing pad 40 is positioned. A drive assembly 26 rotates the platen 20 as indicated by arrow "A". In another existing embodiment, the drive assembly 26 reciprocates the platen back and forth as indicated by arrow "B". The motion of the platen 20 is imparted to the pad 40 because the polishing pad 40 frictionally engages the surface 22 of the platen 20. The wafer carrier 30 has a lower surface 32 to which a wafer 12 may be attached, or the wafer 12 may be attached to a resilient pad 34 positioned between the wafer 12 and the lower surface 32. The wafer carrier 30 may be a weighted, free-floating wafer carrier, or an actuator assembly 36 may be attached to the wafer carrier 30 to impart axial and rotational motion, as indicated by arrows "C" and "D", respectively.
In the operation of the conventional polisher 10, the wafer 12 is positioned face-downward against the polishing pad 40, and then the platen 20 and the wafer carrier 30 move relative to one another. As the face of the wafer 12 moves across the polishing surface 42 of the polishing pad 40, the polishing pad 40 and the slurry 44 remove material from the wafer 12.
CMP processes must consistently and accurately produce a uniform, planar surface on the wafer because it is important to accurately focus circuit patterns on the wafer. As the density of integrated circuits increases, current lithographic techniques must accurately focus the critical dimensions of photo-patterns to within a tolerance of approximately 0.35-0.5 .mu.m. Focusing the photo-patterns to such small tolerances, however, is very difficult when the distance between the emission source and the surface of the wafer varies because the surface of the wafer is not uniformly planar. In fact, when the surface of the wafer is not uniformly planar, several devices on the wafer may be defective. Thus, CMP processes must create a highly uniform, planar surface.
The surface of a wafer, however, may not be uniformly planar because the rate at which the thickness of the wafer decreases as it is being polished (the "polishing rate") often varies from one area on the wafer to another. The polishing rate is a function of several factors, one of which is the local pressure between the pad and the wafer across the face of the wafer. The local pressure between the pad and wafer typically varies because the contour of the polishing surface of the pad may not be uniformly planar. Moreover, the contour the polishing surface of a pad changes over time because one portion of the pad may wear at a different rate than another. For example, in polishing pads made from a polymeric matrix material and an abrasive filler material, regions on the pad with a high density of the filler material wear at a different rate than other regions on the pad. Therefore, it is desirable to measure the contour of the pad throughout the CMP process, and then either condition the pad to enhance the planarity of the pad or adjust the pressure between the wafer and the pad.
One existing device for measuring the contour of the polishing surface of a polishing pad is an arm-type stylus with a needle-like tip attached to a pivotable arm. In operation, the tip follows the contour of the pad as the stylus moves across the surface of the pad. The tip causes the arm to pivot about its pivot point so that the angular deflection of the arm is proportional to the change in the contour of the pad. Another existing device for measuring the contour of the polishing surface is an interferometer. Interferometers typically direct a laser beam at the surface of the wafer and measure a phase change between the original beam and the beam reflected from the polishing surface. By knowing the wavelength of the laser beam, the phase change indicates the linear displacement from one point on the pad to another.
One problem with existing measuring devices is that they are not well suited for accurately measuring the contour of a polishing surface, in real-time while a wafer is polished. Real-time measurements are desirable to eliminate the down-time associated with stopping a polisher to measure the pad. Real-time measurements are also desirable because the contour of a pad may change while a wafer is being polished. However, accurately measuring the contour of the pad in real-time is difficult because the pad is moving and a layer of slurry coats the pad. Interferometers, for example, may generate inaccurate real-time measurements because the light beam may reflect off the slurry instead of the pad. Conventional arm-type styluses may also generate inaccurate real-time measurements because the arm has a relatively large mass compared to the tip. Thus, after the tip passes over a sharp rise on the polishing surface, the upward momentum of the arm may cause the tip to momentarily disengage the pad and produce a false reading.
Another problem with conventional contour measuring devices is that they only measure the contour of the polishing surface of a pad along a radius of the pad. Measuring the contour of the pad along a radius does not provide a highly accurate indication of the contour of the whole pad because the contour along one radius may not be the same as that along another radius. Thus, unless the contour is measured along several radii of the pad (which is time-consuming and inefficient), conventional measuring devices only provide a local measurement of the contour of a polishing pad.
In light of the problems associated with conventional pad contour measuring devices, it would be desirable to develop a device tier measuring the contour of the polishing surface of a polishing pad in real-time while a wafer is being polished. Additionally, it would be desirable to measure a greater area of the polishing pad to provide more data without reducing the throughput of the CMP process.